10 Chapter 10 Transaction Management and Concurrency Control

10 Chapter 10 Transaction Management and Concurrency Control Database Systems: Design, Implementation, and Management, Seventh Edition, Rob and Coronel Database Systems: Design, Implementation, & Management, 7 th Edition, Rob & Coronel

10 In this chapter, you will learn: • What a database transaction is and what its properties are • What concurrency control is and what role it plays in maintaining the database’s integrity • What locking methods are and how they work • How stamping methods are used for concurrency control Database Systems: Design, Implementation, & Management, 7 th Edition, Rob & Coronel 2

10 In this chapter, you will learn (continued): • How optimistic methods are used for concurrency control • How database recovery management is used to maintain database integrity Database Systems: Design, Implementation, & Management, 7 th Edition, Rob & Coronel 3

10. 1 What is a Transaction? 10 Database Systems: Design, Implementation, & Management, 7 th Edition, Rob & Coronel 4

10 What is a Transaction? (continued) • Any action that reads from and/or writes to a database may consist of: – Simple SELECT statement to generate list of table contents – Series of related UPDATE statements to change values of attributes in various tables – Series of INSERT statements to add rows to one or more tables – Combination of SELECT, UPDATE, and INSERT statements Database Systems: Design, Implementation, & Management, 7 th Edition, Rob & Coronel 5

10 What is a Transaction? (continued) • Transaction is logical unit of work that must be either entirely completed or aborted • Successful transaction changes database from one consistent state to another – One in which all data integrity constraints are satisfied • Most real-world database transactions are formed by two or more database requests – A request is equivalent of a single SQL statement in an application program or transaction Database Systems: Design, Implementation, & Management, 7 th Edition, Rob & Coronel 6

Evaluating Transaction Results 10 • Not all transactions update database • SQL code represents a transaction because database was accessed • Improper or incomplete transactions can have devastating effect on database integrity – Some DBMSs provide means by which user can define enforceable constraints – Other integrity rules are enforced automatically by the DBMS • No semantic checking Database Systems: Design, Implementation, & Management, 7 th Edition, Rob & Coronel 7

Example Transaction 10 • INSERT INTO INVOICE VALUES (1009, 10016, ’ 18 -JAN-2006’, 256. 99, 20. 56, 277. 55, ‘cred’, 0. 00, 277. 55) • INSERT INTO LINE VALUES (1009, 1, ’ 89 -WRE-Q’, 1, 256. 99) • UPDATE PRODUCT SET PROD_QOH=PROD_QOH – 1 WHERE PROD_CODE=’ 89 -WRE-Q’ • UPDATE CUSTOMER SET CUS_BALANCE = CUS_BALANCE + 277. 55 WHERE CUS_NUMBER=10016 • INSERT INTO ACCT_TRANSACTION VALUES (10007, ’ 18 -Jan -06’, 10016, ‘charge’, 277. 55) • COMMIT Database Systems: Design, Implementation, & Management, 7 th Edition, Rob & Coronel 8

Evaluating Transaction Results (continued) 10 Figure 9. 2 Database Systems: Design, Implementation, & Management, 7 th Edition, Rob & Coronel 9

10 Transaction Properties • Atomicity – Requires that all operations (SQL requests) of a transaction be completed. If not, the transaction is aborted. • Consistency – Indicates the permanence of database’s consistent state Database Systems: Design, Implementation, & Management, 7 th Edition, Rob & Coronel 10

10 Transaction Properties (continued) • Isolation – Data used during execution of a transaction cannot be used by second transaction until first one is completed • Durability – Once transaction changes are done (committed), they cannot be undone or lost, even in the event of a system failure • Serializability – Ensures that concurrent execution of several transactions yields consistent results Database Systems: Design, Implementation, & Management, 7 th Edition, Rob & Coronel 11

10 Transaction Management with SQL • ANSI has defined standards that govern SQL database transactions • Transaction support is provided by two SQL statements: COMMIT and ROLLBACK Database Systems: Design, Implementation, & Management, 7 th Edition, Rob & Coronel 12

10 Transaction Management with SQL (continued) • ANSI standards require that, when a transaction sequence is initiated by a user or an application program, it must continue through all succeeding SQL statements until one of four events occurs – COMMIT statement is reached – ROLLBACK statement is reached – End of program is reached – Program is abnormally terminated Database Systems: Design, Implementation, & Management, 7 th Edition, Rob & Coronel 13

10 The Transaction Log • Transaction log stores: – A record for the beginning of transaction – For each transaction component (SQL statement): • Type of operation being performed (update, delete, insert) • Names of objects affected by transaction • “Before” and “after” values for updated fields • Pointers to previous and next transaction log entries for the same transaction – Ending (COMMIT) of the transaction Database Systems: Design, Implementation, & Management, 7 th Edition, Rob & Coronel 14

10 The Transaction Log (continued) Database Systems: Design, Implementation, & Management, 7 th Edition, Rob & Coronel 15

10 10. 2 Concurrency Control • Coordination of simultaneous transaction execution in a multiprocessing database system • Objective is to ensure serializability of transactions in a multiuser database environment Database Systems: Design, Implementation, & Management, 7 th Edition, Rob & Coronel 16

10 Concurrency Control (continued) • Simultaneous execution of transactions over a shared database can create several data integrity and consistency problems – Lost updates – Uncommitted data – Inconsistent retrievals Database Systems: Design, Implementation, & Management, 7 th Edition, Rob & Coronel 17

10 Lost Updates Database Systems: Design, Implementation, & Management, 7 th Edition, Rob & Coronel 18

10 Lost Updates (continued) Database Systems: Design, Implementation, & Management, 7 th Edition, Rob & Coronel 19

10 Uncommitted Database Systems: Design, Implementation, & Management, 7 th Edition, Rob & Coronel 20

10 Uncommitted Data (continued) Database Systems: Design, Implementation, & Management, 7 th Edition, Rob & Coronel 21

10 Inconsistent Retrievals Database Systems: Design, Implementation, & Management, 7 th Edition, Rob & Coronel 22

10 Inconsistent Retrievals (continued) Database Systems: Design, Implementation, & Management, 7 th Edition, Rob & Coronel 23

10 Inconsistent Retrievals (continued) Database Systems: Design, Implementation, & Management, 7 th Edition, Rob & Coronel 24

10 The Scheduler • As long as two transactions access unrelated data, there is no conflict in the execution order is irrelevant to the final outcome. • The scheduler is a special DBMS program – Purpose is to establish order of operations within which concurrent transactions are executed • Interleaves execution of database operations to ensure serializability and isolation of transactions Database Systems: Design, Implementation, & Management, 7 th Edition, Rob & Coronel 25

10 The Scheduler (continued) • Bases its actions on concurrency control algorithms • Ensures computer’s central processing unit (CPU) is used efficiently – First-come first-served scheduling wastes processing time when CPU waits for READ or WRITE operation • Facilitates data isolation to ensure that two transactions do not update same data element at same time Database Systems: Design, Implementation, & Management, 7 th Edition, Rob & Coronel 26

10 The Scheduler (continued) for the same data unit Database Systems: Design, Implementation, & Management, 7 th Edition, Rob & Coronel 27

10 10. 3 Concurrency Control with Locking Methods • Lock – Guarantees exclusive use of a data item to a current transaction – Required to prevent another transaction from reading inconsistent data • Lock manager – Responsible for assigning and policing the locks used by transactions Database Systems: Design, Implementation, & Management, 7 th Edition, Rob & Coronel 28

10 Lock Granularity • Indicates level of lock use • Locking can take place at following levels: – Database – Table – Page – Row – Field (attribute) Database Systems: Design, Implementation, & Management, 7 th Edition, Rob & Coronel 29

10 Lock Granularity (continued) • Database-level lock – Entire database is locked • Table-level lock – Entire table is locked • Page-level lock – Entire diskpage is locked Database Systems: Design, Implementation, & Management, 7 th Edition, Rob & Coronel 30

10 Lock Granularity (continued) • Row-level lock – Allows concurrent transactions to access different rows of same table, even if rows are located on same page • Field-level lock – Allows concurrent transactions to access same row, as long as they require use of different fields (attributes) within that row Database Systems: Design, Implementation, & Management, 7 th Edition, Rob & Coronel 31

Lock Granularity (continued) Database Systems: Design, Implementation, & Management, 7 th Edition, Rob & Coronel 10 32

Lock Granularity (continued) Database Systems: Design, Implementation, & Management, 7 th Edition, Rob & Coronel 10 33

Lock Granularity (continued) Database Systems: Design, Implementation, & Management, 7 th Edition, Rob & Coronel 10 34

10 Lock Granularity (continued) Database Systems: Design, Implementation, & Management, 7 th Edition, Rob & Coronel 35

Lock Types 10 • Binary lock – Has only two states: locked (1) or unlocked (0) – Every transaction requires a lock and unlock operation for each accessed data item, which are automatically managed by the DBMS • Exclusive lock – Access is specifically reserved for transaction that locked object – Mutual exclusive rule – Must be used when potential for conflict exists • Shared lock – Concurrent transactions are granted. Read access on basis a Implementation, common lock 7 Edition, Rob & Coronel 36 Database Systems: of Design, & Management, th

10 Lock Types (continued) Database Systems: Design, Implementation, & Management, 7 th Edition, Rob & Coronel 37

10 Two-Phase Locking to Ensure Serializability • Defines how transactions acquire (得到) and relinquish (放棄) locks • Guarantees serializability, but it does not prevent deadlocks – Growing phase - Transaction acquires all required locks without unlocking any data – Shrinking phase - Transaction releases all locks and cannot obtain any new lock Database Systems: Design, Implementation, & Management, 7 th Edition, Rob & Coronel 38

10 Two-Phase Locking to Ensure Serializability (continued) • Governed by the following rules: – Two transactions cannot have conflicting locks – No unlock operation can precede a lock operation in the same transaction – No data are affected until all locks are obtained—that is, until transaction is in its locked point Database Systems: Design, Implementation, & Management, 7 th Edition, Rob & Coronel 39

Two-Phase Locking to Ensure Serializability (continued) Database Systems: Design, Implementation, & Management, 7 th Edition, Rob & Coronel 10 40

10 Deadlocks • Condition that occurs when two transactions wait for each other to unlock data • Possible only if one of the transactions wants to obtain an exclusive lock on a data item – No deadlock condition can exist among shared locks Database Systems: Design, Implementation, & Management, 7 th Edition, Rob & Coronel 41

10 Deadlocks (continued) • Control through: – Prevention – Detection – Avoidance Database Systems: Design, Implementation, & Management, 7 th Edition, Rob & Coronel 42

Deadlocks (continued) Database Systems: Design, Implementation, & Management, 7 th Edition, Rob & Coronel 10 43

10 10. 4 Concurrency Control with Time Stamping Methods • Assigns global unique time stamp to each transaction • Produces explicit order in which transactions are submitted to DBMS • Uniqueness – Ensures that no equal time stamp values can exist • Monotonicity – Ensures that time stamp values always increase Database Systems: Design, Implementation, & Management, 7 th Edition, Rob & Coronel 44

10 Wait/Die and Wound/Wait Schemes • Wait/die – Older requesting transaction waits – Younger requesting transaction is rolled back and rescheduled • Wound/wait – Older requesting transaction preempts (rolls back) younger transaction and reschedules it – Younger requesting transaction waits Database Systems: Design, Implementation, & Management, 7 th Edition, Rob & Coronel 45

10 Wait/Die and Wound/Wait Schemes (continued) Database Systems: Design, Implementation, & Management, 7 th Edition, Rob & Coronel 46

10 10. 5 Concurrency Control with Optimistic Methods • Optimistic approach – Based on assumption that majority of database operations do not conflict – Does not require locking or time stamping techniques – Transaction is executed without restrictions until it is committed – Phases are read, validation, and write Database Systems: Design, Implementation, & Management, 7 th Edition, Rob & Coronel 47

10 10. 6 Database Recovery Management • Database recovery – Restores database from given state, usually inconsistent, to previously consistent state – Based on atomic transaction property • All portions of transaction must be treated as single logical unit of work, so all operations must be applied and completed to produce consistent database – If transaction operation cannot be completed, transaction must be aborted, and any changes to database must be rolled back (undone) Database Systems: Design, Implementation, & Management, 7 th Edition, Rob & Coronel 48

10 Concepts that Affect Transaction Recovery • Write-ahead protocol • Redundant transaction logs • Database buffers • Database checkpoints Database Systems: Design, Implementation, & Management, 7 th Edition, Rob & Coronel 49

10 Transaction Recovery • Makes use of deferred-write and write-through techniques • Deferred write – Transaction operations do not immediately update physical database – Only transaction log is updated – Database is physically updated only after transaction reaches its commit point using transaction log information Database Systems: Design, Implementation, & Management, 7 th Edition, Rob & Coronel 50

10 Transaction Recovery (continued) • Write-through or immediate-update – • Database is immediately updated by transaction operations during transaction’s execution, even before transaction reaches its commit point Recovery process for write-through 1. Identify the last checkpoint 2. Do nothing for transactions started and committed before the last checkpoint 3. Redo transactions committed after the last checkpoint 4. Undo transactions that had a ROLLBACK operation after the last checkpoint or that was left active before the failure occurred Database Systems: Design, Implementation, & Management, 7 th Edition, Rob & Coronel 51

10 Transaction Recovery (continued) • Recovery process for deferred-write 1. Identify the last checkpoint 2. Do nothing for transactions started and committed before the last checkpoint 3. Redo transactions committed after the last checkpoint 4. Any other transactions will be ignored Database Systems: Design, Implementation, & Management, 7 th Edition, Rob & Coronel 52

10 Transaction Recovery (continued) Database Systems: Design, Implementation, & Management, 7 th Edition, Rob & Coronel 53

10 Summary • Transaction – Sequence of database operations that access database – Represents real-world events – Must be logical unit of work • No portion of transaction can exist by itself – Takes database from one consistent state to another • One in which all data integrity constraints are satisfied Database Systems: Design, Implementation, & Management, 7 th Edition, Rob & Coronel 54

10 Summary (continued) • Transactions have five main properties: atomicity, consistency, isolation, durability, and serializability • SQL provides support for transactions through the use of two statements: COMMIT and ROLLBACK • SQL transactions are formed by several SQL statements or database requests Database Systems: Design, Implementation, & Management, 7 th Edition, Rob & Coronel 55

10 Summary (continued) • Transaction log keeps track of all transactions that modify database • Concurrency control coordinates simultaneous execution of transactions • Scheduler is responsible for establishing order in which concurrent transaction operations are executed Database Systems: Design, Implementation, & Management, 7 th Edition, Rob & Coronel 56

10 Summary (continued) • Lock guarantees unique access to a data item by transaction • Two types of locks can be used in database systems: binary locks and shared/exclusive locks • Serializability of schedules is guaranteed through the use of two-phase locking Database Systems: Design, Implementation, & Management, 7 th Edition, Rob & Coronel 57

10 Summary (continued) • When two or more transactions wait indefinitely for each other to release lock, they are in deadlock, or deadly embrace • Three deadlock control techniques: prevention, detection, and avoidance Database Systems: Design, Implementation, & Management, 7 th Edition, Rob & Coronel 58

10 Summary (continued) • Concurrency control with time stamping methods assigns unique time stamp to each transaction and schedules execution of conflicting transactions in time stamp order Database Systems: Design, Implementation, & Management, 7 th Edition, Rob & Coronel 59

10 Summary (continued) • Concurrency control with optimistic methods assumes that the majority of database transactions do not conflict and that transactions are executed concurrently, using private copies of the data • Database recovery restores database from given state to previous consistent state Database Systems: Design, Implementation, & Management, 7 th Edition, Rob & Coronel 60